Stack emissions

In magnesium casting, sulfur dioxide is employed as an inert blanketing gas. Another foundry appHcation is as a rapid curing catalyst for furfuryl resins in cores. Surprisingly, in view of the many efforts to remove sulfur dioxide from flue gases, there are situations where sulfur dioxide is deHberately introduced. In power plants burning low sulfur coal and where particulate stack emissions are a problem, a controUed amount of sulfur dioxide injection improves particulate removal. 

In early years the contact process frequentiy employed only two or three catalyst stages (passes) to obtain overall SO2 conversions of approximately 95—96%. Later, four pass converters were used to obtain conversions of from 97% to slightiy better than 98%. For sulfur-burning plants, this typically resulted in sulfur dioxide stack emissions of 1500—2000 ppm. 

Small amounts of sulfuric acid mist or aerosol are always formed in sulfuric acid plants whenever gas streams are cooled, or SO and H2O react, below the sulfuric acid dew point. The dew point varies with gas composition and pressure but typically is 80—170°C. Higher and lower dew point temperatures are possible depending on the SO concentration and moisture content of the gas. Such mists are objectionable because of both corrosion in the process and stack emissions. 

More recentiy, sulfuric acid mists have been satisfactorily controlled by passing gas streams through equipment containing beds or mats of small-diameter glass or Teflon fibers. Such units are called mist eliminators (see Airpollution control methods). Use of this type of equipment has been a significant factor in making the double absorption process economical and in reducing stack emissions of acid mist to tolerably low levels. 

Double-Absorption Plants. In the United States, newer sulfuric acid plants ate requited to limit SO2 stack emissions to 2 kg of SO2 per metric ton of 100% acid produced (4 Ib /short ton Ib = pounds mass). This is equivalent to a sulfur dioxide conversion efficiency of 99.7%. Acid plants used as pollution control devices, for example those associated with smelters, have different regulations. This high conversion efficiency is not economically achievable by single absorption plants using available catalysts, but it can be attained in double absorption plants when the catalyst is not seriously degraded. 

Exhaust emissions of CO, unbumed hydrocarbons, and nitrogen oxides reflect combustion conditions rather than fuel properties. The only fuel component that degrades exhaust is sulfur the SO2 concentrations ia emissions are directly proportional to the content of bound sulfur ia the fuel. Sulfur concentrations ia fuel are determined by cmde type and desulfurization processes. Specifications for aircraft fuels impose limits of 3000 —4000 ppm total sulfur but the average is half of these values. Sulfur content ia heavier fuels is determined by legal limits on stack emissions. 

Cement plants in the United States are now carehiUy monitored for compliance with Environmental Protection Agency (EPA) standards for emissions of particulates, SO, NO, and hydrocarbons. AH plants incorporate particulate collection devices such as baghouses and electrostatic precipitators (see Air POLLUTION CONTROL methods). The particulates removed from stack emissions are called cement kiln dust (CKD). It has been shown that CKD is characterized by low concentrations of metals which leach from the CKD at levels far below regulatory limits (63,64). Environmental issues continue to be of concern as the use of waste fuel in cement kilns becomes more widespread. 

FIG. 25-6 Lapse-rate characteristics of atmospheric-diffusion transport of stack emissions. 

This method is used for the determination of total chromium (Cr), cadmium (Cd), arsenic (As), nickel (Ni), manganese (Mn), beiylhum (Be), copper (Cu), zinc (Zn), lead (Pb), selenium (Se), phosphorus (P), thalhum (Tl), silver (Ag), antimony (Sb), barium (Ba), and mer-cuiy (Hg) stack emissions from stationaiy sources. This method may also be used for the determination of particulate emissions fohowing the procedures and precautions described. However, modifications to the sample recoveiy and analysis procedures described in the method for the purpose of determining particulate emissions may potentially impacl the front-half mercury determination. 

Mean wind speed and direction The air flow is assumed to be horizontal, but the flow may be tilted (to yield a vertical component) due to local topographic effects. The mean wind speed determines the convection of the stack emissions. 

For example, if 40 percent of stack emissions of the reported substance were derived using monitoring data, 30 percent by mass balance, and 30 percent by emission factors, you would enter the code letter M" for monitoring. 

Screen oriented, menu driven program that facilitates data editing, data analysis and preparation of reports for stack emissions. 

Any stack should be designed based on a knowledge of prevailing meteorological conditions and stack emission criteria based on years of operating... 

We shall now provide a second example to illustrate step-by-step calculations. In this example a flare stack is estimated to be 80% efficient in combusting HjS off-gas. The total off-gas through the stack is 400,000 kg/hr, of which 7.0 weight percent is H2S. The physieal stack height is 250 m, the stack diameter is 5.5 m, and the stack emission velocity is 18 m/s. The stack emission temperature is 15°C. The meteorological conditions may be described as a bright sunny day with a mean wind speed of 3 m/s. 

The hazards associated with normal plant operations, such as normal stack emissions and fugitive emissions, as well as those resulting from specific incidents such as spills, leaks, fires and explosions should be considered. 

Stack emissions can include particulates as well as dense gases (heavier tlian air, e.g., chlorine). These are subjected to a downwash settling tlirough tlie alinosphere due to tlie action of gravity. For tlie particles, especially large ones, an additional external force term must be included in the analysis. 

CRSTER estimates ground-level concentrations resulting from up to 19 colocated elevated stack emissions. 

One way to remove nitrogen oxide (NO) from smoke stack emissions is to react it with ammonia. 

The presence of asphaltenes, originating in the fuel, acts as a trap for vanadium, nickel, and sodium (which promote slagging and sulfur corrosion)-, these asphalthenes often contain sulfur compounds, which simply add to the contaminant load. Additionally, asphaltenes act as precursors to spherical stack solids (cenospheres), which are exhausted with the flue gases as stack emissions. 

Here a chemical reaction produces a molecule with electrons in an excited state. Upon decay to the ground state the liberated radiation is detected. One such example is the reaction between ozone and nitric oxide to form nitrogen dioxide emitting radiation in the near infra-red in the 0.5-3/t region. The technique flnds use for measuring nitric oxide in ambient air or stack emissions. 

Release of trichloroethylene also occurs at treatment and disposal sites. Water treatment facilities may release trichloroethylene from contaminated water through volatilization and air-stripping procedures (EPA 1985e). Trichloroethylene is also released to the atmosphere through gaseous emissions from landfills. The compound may occur as either an original contaminant or as a result of the decomposition of tetrachloroethylene. Trichloroethylene has also been detected in stack emissions from the incineration of municipal and hazardous waste (James et al. 1985 Oppelt 1987). 

Are all emissions and discharges documented in an inventory, for example, process effluent domestic wastewater, cooling water, stack emissions, hazardous wastes, nonhazardous wastes Provide a schedule of emissions. Identify the risk category. 

Maximum allowable carbon monoxide levels in stack emissions... 

The final emission standard under the BIF regulations limits the unit s output of HCI and chlorine gas (Cl j). These compounds combine with water in the air to form acid rain. They are also a known cause of human respiratory problems. The emission controls are implemented in the same way as the metal emissions, using the tiered approach. The owner/operator has a choice of three tiers with varying focal points. The Tier I and Tier II screening levels for waste feed and stack emission limits are located in Part 266, Appendices II and III.5... 

U.S. EPA s recommendations regarding stack emission tests, which may be performed at hazardous waste combustion facilities for the purpose of supporting MACT standards and multipathway, site-specific risk assessments, where such a risk assessment has been determined to be necessary by the permit authority, can be found in the U.S. EPA document on Risk Burn Guidance for Hazardous Waste Combustion Facilities.32 The applicability of the new standards has been demonstrated in the management of hazardous waste incinerators, whose performance was shown to clearly surpass the regulatory requirements in all tested areas.33... 

There are also several possibilities for the temporal distribution of releases. Although some releases, such as those stemming from accidents, are best described as instantaneous release of a total amount of material (kg per event), most releases are described as rates kg/sec (point source), kg/sec-m (line source), kg/sec-m (area source). (Note here that a little dimensional analysis will often indicate whether a factor or constant in a fate model has been inadvertently omitted.) The patterns of rates over time can be quite diverse (see Figure 3). Many releases are more or less continuous and more or less uniform, such as stack emissions from a base-load power plant. Others are intermittent but fairly regular, or at least predictable, as when a coke oven is opened or a chemical vat... 

Some identified modes are evaporation through a stack emission through a vent (a vent is not designed to elevate the emitted material—a stack is) leaks in plumbing or storage containers and wind-blown dust. 

Chu, P and D.B. Porcella. 1995. Mercury stack emissions from U.S. electric utility power plants. Water Air Soil Pollut. 80 135-144. 

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